1. Field of the Invention
The present invention relates to display elements in which light emission is controlled by the amount of current flowing through an electro-optical element, as well as to display devices including such display elements.
2. Description of the Related Art
Conventionally, there are display elements using, as electro-optical elements, elements whose light emission amount is controlled by the amount of current flowing through the elements, for example LEDs (light emitting diodes) for which inorganic EL (electroluminescent) elements and organic EL elements are typical examples. In this specification, “electro-optical elements” mean any elements whose optical characteristics are changed by applying electricity, such as not only the aforementioned organic EL elements etc., but also FEDs (field emission displays), charge-driven elements, liquid crystals, E-ink (electronic ink) and the like. It should be noted that in the following, organic EL elements are taken as an example of electro-optical elements, but the same explanations are also possible for any light emitting element whose light emission amount is controlled by a current amount.
Furthermore, there are display devices, in which a plurality of display elements are arranged in a matrix, such that pixels are formed with the display elements. Each display element includes a switching element, which controls the amount of current flowing through the electro-optical element, in accordance with an external electrical signal. Examples of such switching elements are diodes and MIM (metal insulator metal) elements, but more preferable are thin film transistors (referred to in short as “TFTs” in the following) using amorphous silicon or polycrystalline silicon, which has better switching characteristics, continuous grain silicon (referred to in short as “CG silicon” in the following), which is a polycrystalline silicone with higher crystallinity, or an organic material film having a conductivity that is different from that of silicon films. Depending on the form of the electrical signals controlling the amount of current flowing through such an electro-optical element, the control methods can be broadly classified into two methods, namely the constant voltage control method and the constant current control method, which are described below.
First, the constant voltage control method is described. FIG. 12 is a diagram showing an equivalent circuit of a display element forming one pixel. This display element includes an organic EL element 920, which is an electro-optical element, a power source line electrode 912, which supplies current from a driving power source Vref, a scanning signal line electrode 910, a data signal line electrode 911, a common electrode Vcom, an auxiliary capacitance 930, a current control TFT 92, which is a p-channel TFT for controlling the current flowing through the organic EL element 920, and a data voltage control TFT 91, which is an n-channel TFT for controlling the timing at which current flows through the organic EL element 920.
As shown in FIG. 12, the source terminal of the current control TFT 92 and one side of the auxiliary capacitance 930 are connected to the power source line electrode 912, whereas the other side of the auxiliary capacitance 930 is connected to the gate terminal of the current control TFT 92 and the drain terminal of the data voltage control TFT 91. Moreover, the source terminal of the data voltage control TFT 91 is connected to the data signal line electrode 911, and the gate terminal of the data voltage control TFT 91 is connected to the scanning signal line electrode 910. Furthermore, the anode of the organic EL element 920 is connected to the drain terminal of the current control TFT 92, whereas the cathode of the organic EL element 920 is connected to the common electrode Vcom. With the constant voltage control method, by applying a data signal voltage to the data signal line electrode 911 in the period in which the data voltage control TFT 91 is selected by a scanning signal applied to the scanning signal line electrode 910, a voltage corresponding to that data signal voltage is held by the auxiliary capacitance 930. After that, in the period in which the data voltage control TFT 91 is not selected, the conductance of the current control TFT 92 is controlled in accordance with the voltage held by the auxiliary capacitance 930, so that a predetermined current flows through the organic EL element 920, which is connected in series to this current control TFT 92, thus controlling the light emission amount of the organic EL element 920. This configuration is described, for example, in “Active Matrix Addressing of Polymer Light Emitting Diodes Using Low Temperature Poly Silicon TFTs” by I. M. Hunter et. al., AM-LCD2000, 2000, pp. 249 to 252.
Moreover, in order to control the light emission amount and to achieve a display device with high image quality, it is preferable that the potential that is held by the auxiliary capacitance is changed discretely by making this data signal a digital signal, and it is known that for this, it is preferable that the ON resistance of the current control TFT is negligibly small compared to the ON resistance of the light emitting pixel. Such a configuration is described for example in JP H11-73158A.
In contrast to this constant voltage control method, in the constant current control method, a voltage with which the current is obtained that is supposed to flow through the organic EL element 920 is held by the auxiliary capacitance, due to the data signal current that flows through the data signal line electrode of the display element shown in FIG. 12. The light emission amount of the organic EL element 920 is controlled through the voltage held by this auxiliary capacitance 930. The following is a more specific explanation of a configuration for letting the current flow that is supposed to flow through the display element.
FIG. 13 is a diagram showing an equivalent circuit of a conventional example of a display element according to the constant current control method. As shown in FIG. 13, a constant current circuit is connected to the data signal line electrode 911, and TFTs 93 and 94 as well as a signal line electrode 940 for controlling the TFT 94 are further provided in the display element. Like the TFT 91, the TFT 93 is controlled by the scanning signal line electrode 910.
First, when the TFTs 91 and 93 are ON, and the TFT 94 is OFF, a current flows from the driving power source Vref through the TFT 93 to the constant current circuit connected to the data signal line electrode 911, until the current flowing between the source and drain of the TFT 92 takes on a desired value. In this situation, a voltage that is such that a current can flow between source and drain of the TFT 92 is stored in the auxiliary capacitance 930, so that after the TFTs 91 and 93 are switched OFF, the desired current can be caused to flow though the organic EL element 920 by switching the TFT 94 ON. This configuration is described, for example, in “Circuit Simulation of Current-Specified Polysilicon TFT Active Matrix Driven Organic LED Displays,” Reiji Hattori et. al., SHINGAKU GIHO, April 2001, ED 2001-8, SDM 2001-8, Vol. 101, No. 15, pp. 7-14.
The following is a description of the relation between the current flowing through the organic EL element 920 and the light emission amount of the organic EL element 920. FIG. 14 shows the relation between the emitted luminance of an organic EL element and the current flowing at the time of emission. As shown in FIG. 14, the emitted luminance of the organic EL element is substantially proportional to the current flowing through the organic EL element, that is, the driving current of the current control TFT 92 shown in FIG. 12. This relation is generally well known.
In the constant voltage control method, when the internal resistance of the organic EL element increases over time (mainly caused by element deterioration due to reactions between moisture and oxygen, decomposition of the material, and changes in the shape of the film layers), then current flowing through the organic EL element is relatively reduced, because the load of the power source is relatively increased. And thus, since the emitted luminance is proportional to the current flowing at the time of the emission, as shown in FIG. 14, also the emitted luminance of the organic EL element is relatively decreased.
On the other hand, with the constant current control method, the current of the data signal is controlled such that a constant current flows through the organic EL element, regardless of the passage of time (and the increase of the internal resistance over time). Therefore, a reduction of the emitted luminance due to a decrease of the flowing current as described above does not occur. However, ordinary organic EL elements have the characteristic that their emitted luminance for a given current decreases over time (due to deterioration of the element), so that also with the constant current control method, a decrease in emitted luminance occurs over the course of time.
FIG. 15 is a diagram illustrating the decrease in emitted luminance over the course of time in organic EL elements with the constant voltage control method and the constant current control method. Curve A shows the case of the constant voltage control method, whereas curve B shows the case of the constant current control method. Moreover, for the normalized time shown in FIG. 15, the time when the normalized luminance for the constant voltage control method has been reduced by half (i.e. reduced to 0.5) is taken as 1. As shown in FIG. 15, the display lifetime of the display apparatus with the constant current control method is longer than that with the constant voltage control method. It should be noted that this display lifetime means the time until, due to deterioration over time, the elements emitting a predetermined luminance reach a luminance at which they cannot be used anymore for a display device, that is, the time until the aforementioned predetermined luminance has been reduced approximately by half.
This decrease of the emitted luminance due to deterioration over time is subject to variations caused by differences in the emission history of each organic EL element, which causes variations in the characteristics of the TFT elements included in the display panel. To address this problem, it is known to connect the organic EL element in series with a resistance element, to adjust the voltage applied to the organic EL element by voltage division, and to reduce the relative variations in the emitted light. Such a configuration is disclosed in JP 2001-272930A, pages 3 and 4, FIG. 1 and JP 2002-175029A, pages 6-8, FIGS. 2 and 5 to 7.
In this configuration, a predetermined resistance element R is further connected in series to the organic EL element 920 of the display element shown in FIG. 12, in addition to the current control TFT 92. FIG. 16 is a diagram showing an equivalent circuit of a display element according to this conventional example. As shown in FIG. 16, by increasing the internal resistance of the organic EL element 920, even if the current that flows is lowered over time from a predetermined value, it is possible to increase the voltage applied to the organic EL element 920 with the internal resistance of the organic EL element 920, in accordance with the rule of voltage division. Therefore, a reduction of the luminance due to a temporal reduction of the current can be inhibited. Consequently, even for the case of the constant voltage control method, it is possible to realize a display device in which the reduction of the emitted luminance is inhibited, as in the case of the constant current control method.
However, in configurations performing control in accordance with a digital data signal with the constant voltage control method, as in these conventional configurations (see for example the configuration in JP H11-73158A), it is preferable that the resistance of the current control TFT element is so small that it can be ignored, so that in order to increase the display lifetime in this configuration, the above-noted configuration is necessary in which the voltage applied to the organic EL element due to voltage division is increased in accordance with the internal resistance of the organic El element, by connecting a resistance element in series to the organic EL element within the display element. As an alternative method for increasing the display lifetime, it is conceivable to configure a display element with the constant current control method in which a constant current can always be caused to flow through the organic EL element, regardless to the rules of voltage division. However, in such a configuration, the number of switching elements and resistances included in the display element increases compared to the display element shown in FIG. 12. Therefore, the surface area occupied by these increases, so that the proportion of the surface area occupied by those elements to the surface area of the overall display element increases as well. As a result, the light emission area of the light emitting element is reduced, so that the numerical aperture of the display element is reduced.
In order to realize the same display luminance as with the display element of the circuit configuration shown in FIG. 12 with a configuration in which the numerical aperture is reduced, as described above, it is necessary to increase the luminance of the organic EL element included in the display element. However, it is well known that when the luminance of the organic EL element is stepped up, then the deterioration of the organic EL element occurs even earlier, thus shortening the display lifetime. Moreover, the light emission efficiency of the organic EL element is reduced, and also the power consumption of the organic EL element is relatively increased, so that also in view of the display characteristics, it is not preferable to step up the luminance of the organic EL element.
It should be noted that the explanations here relate to an organic EL element as an example of an electro-optical element, but there is no limitation to organic EL elements, and similar problems occur also with other light emitting elements in which the light emission amount of the element is controlled in accordance with the current amount flowing through the element.